1. Field of the Invention
The present invention is broadly concerned with novel superconducting bulk materials made up of precursor superconducting ceramic oxide crystals (either single or multiple phase) with substantially non-reactive elemental metal in the interstices between ceramic crystals so that the composite materials include respective superconducting and continuous metallic matrices. These materials exhibit desirable ductilities, the ability to carry high D.C. voltage-induced transport critical current densities, and enhanced magnetic properties. The resulting composite materials have T.sub.c values not substantially depressed as compared with the T.sub.c values of the precursor superconducting ceramic oxides, in the absence of an elemental metal matrix. The invention also comprehends the method of fabricating the ceramic oxide/elemental metal superconductors, wherein the ceramic oxide is first fabricated to completion, followed by grinding, mixing with the elemental metal powder, high pressure compaction of the mixture and final sintering.
2. Description of the Prior Art
Superconductivity refers to that special state of a material where its resistance to electrical current flow suddenly and completely disappears when its temperature is lowered. Below this onset or critical temperature T.sub.c, a characteristic of the material, the electrical resistance does not merely drop to a low level but it vanishes entirely. Only a very limited list of materials exhibit such a state. The discovery of the first superconductor occurred in 1911. Heike Kammerlingh Onnes discovered that Mercury lost all detectable resistance at a temperature just 4.degree. above absolute zero.
A superconductor also exhibits perfect diamagnetism below its critical temperature, i.e., it expels all magnetic field lines from its interior by producing an opposing magnetic field from a current flowing on its surface. As a consequence of the perfect diamagnetism of superconductors, they can be used to produce magnetic levitation as envisioned in high speed transport systems of the future, where magnetic repulsion is used to counter gravity. The perfect diamagnetism property of superconductors is called the Meissner effect after its discoverer.
Superconductivity is the only large scale quantum phenomenon involving charges found in solid materials. The current-carrying electrons in the superconductor behave as if they were part of a monumentally large single molecule the size of the entire specimen of the material. The macroscopic quantum nature of superconductors makes them useful in measuring magnetic field quantities to high precision or facilitates the measurement of such quantities so small as to be heretofore unmeasurable.
Despite the tremendous potential of superconductors, and the extensive research heretofore undertaken to develop these materials, a number of formidable problems remain. For example, ceramic oxide superconductors are in general very brittle, and this is also true for the most desirable high T.sub.c superconductors. Thus, while there exists a wide range of ductilities between different families of high T.sub.c ceramic oxide superconductors (e.g., the bismuth-vanadium family is more ductile than the rare earth or thallium families),in general it is very difficult to drill, saw or otherwise mechanically reduce ceramic superconductors without cracking or breakage thereof.
In addition, the crystalline structure of prior ceramic superconductors can be difficult to create, and numerous intergranular weak links are typically present. These weak links can be viewed as S-I-S (superconducting-insulating-superconducting) junctions and tend to substantially lessen the superconducting current densities that can be carried by these materials. Finally, all ceramic oxide superconductors are Type II superconductors. As such, quantized magnetic fluxoids are formed in the presence of an external magnetic field. The ceramic superconductors have relatively large penetration depths, and therefore the presence of large arrays of fluxoids will decrease the electro-magnetic shielding properties of these materials. Moreover, the lack of pinning centers allows for easy merging of the fluxoids, thus quenching superconductivity characteristics altogether.